Ginkgo (Ginkgo biloba; in Chinese and Japanese 銀杏, pinyin romanization: yín xìng, Hepburn romanization: ichō or ginnan), also spelled gingko and known as the Maidenhair Tree, is a unique species of tree with no close living relatives. The tree is widely cultivated and introduced, since an early period in human history, and has various uses as a food and traditional medicine.

Description

Ginkgos are large trees, normally reaching a height of 20–35 m (66–115 feet), with some specimens in China being over 50 m (164 feet). The tree has an angular crown and long, somewhat erratic branches, and is usually deep rooted and resistant to wind and snow damage. Young trees are often tall and slender, and sparsely branched; the crown becomes broader as the tree ages. During autumn, the leaves turn a bright yellow, then fall, sometimes within a short space of time (1–15 days). A combination of resistance to disease, insect-resistant wood and the ability to form aerial roots and sprouts makes ginkgos long-lived, with some specimens claimed to be more than 2,500 years old.

Ginkgo is a relatively shade-intolerant species that (at least in cultivation) grows best in environments that are well-watered and well-drained. The species shows a preference for disturbed sites; in the "semi-wild" stands at Tian Mu Shan, many specimens are found along stream banks, rocky slopes, and cliff edges. Accordingly, Ginkgo retains a prodigious capacity for vegetative growth. It is capable of sprouting from embedded buds near the base of the trunk (lignotubers, or basal chi chi) in response to disturbances, such as soil erosion. Old individuals are also capable of producing aerial roots (chi chi) on the undersides of large branches in response to disturbances such as crown damage; these roots can lead to successful clonal reproduction upon contacting the soil. These strategies are evidently important in the persistence of Ginkgo; in a survey of the "semi-wild" stands remaining in Tian Mu Shan, 40% of the Ginkgo specimens surveyed were multi-stemmed, and few saplings were present.[3]
[edit] Stem

Ginkgo branches grow in length by growth of shoots with regularly spaced leaves, as seen on most trees. From the axils of these leaves, "spur shoots" (also known as short shoots) develop on second-year growth. Short shoots have very short internodes (so they may grow only one or two centimeters in several years) and their leaves are usually unlobed. They are short and knobby, and are arranged regularly on the branches except on first-year growth. Because of the short internodes, leaves appear to be clustered at the tips of short shoots, and reproductive structures are formed only on them (see pictures below - seeds and leaves are visible on short shoots). In Ginkgos, as in other plants that possess them, short shoots allow the formation of new leaves in the older parts of the crown. After a number of years, a short shoot may change into a long (ordinary) shoot, or vice versa.
Leaves

The leaves are unique among seed plants, being fan-shaped with veins radiating out into the leaf blade, sometimes bifurcating (splitting) but never anastomosing to form a network.[4] Two veins enter the leaf blade at the base and fork repeatedly in two; this is known as dichotomous venation. The leaves are usually 5–10 cm (2-4 inches), but sometimes up to 15 cm (6 inches) long. The old popular name "Maidenhair tree" is because the leaves resemble some of the pinnae of the maidenhair fern, Adiantum capillus-veneris.

Leaves of long shoots are usually notched or lobed, but only from the outer surface, between the veins. They are borne both on the more rapidly-growing branch tips, where they are alternate and spaced out, and also on the short, stubby spur shoots, where they are clustered at the tips.
Reproduction

Ginkgos are dioecious, with separate sexes, some trees being female and others being male. Male plants produce small pollen cones with sporophylls each bearing two microsporangia spirally arranged around a central axis.

Female plants do not produce cones. Two ovules are formed at the end of a stalk, and after pollination, one or both develop into seeds. The seed is 1.5–2 cm long. Its fleshy outer layer (the sarcotesta) is light yellow-brown, soft, and fruit-like. It is attractive in appearance, but contains butanoic acid[5] (also known as butyric acid) and smells like rancid butter or vomit[6] when fallen. Beneath the sarcotesta is the hard sclerotesta (the "shell" of the seed) and a papery endotesta, with the nucellus surrounding the female gametophyte at the center.[7]

The fertilization of ginkgo seeds occurs via motile sperm, as in cycads, ferns, mosses and algae. The sperm are large (about 70–90 micrometres)[8] and are similar to the sperm of cycads, which are slightly larger. Ginkgo sperm were first discovered by the Japanese botanist Sakugoro Hirase in 1896.[9] The sperm have a complex multi-layered structure, which is a continuous belt of basal bodies that form the base of several thousand flagella which actually have a cilia-like motion. The flagella/cilia apparatus pulls the body of the sperm forwards. The sperm have only a tiny distance to travel to the archegonia, of which there are usually two or three. Two sperm are produced, one of which successfully fertilizes the ovule. Although it is widely held that fertilization of ginkgo seeds occurs just before or after they fall in early autumn,[4][7] embryos ordinarily occur in seeds just before and after they drop from the tree.[10]
Distribution and habitat

Although Ginkgo biloba and other species of the genus were once widespread throughout the world, their range shrank until by two million years ago it was restricted to a small area of China. For centuries it was thought to be extinct in the wild, but is now known to grow in at least two small areas in Zhejiang province in Eastern China, in the Tian Mu Shan Reserve. However, recent studies indicate high genetic uniformity among ginkgo trees from these areas, arguing against a natural origin of these populations and suggesting that the ginkgo trees in these areas may have been planted and preserved by Chinese monks over a period of about 1,000 years.[11] Whether native ginkgo populations still exist has not been demonstrated unequivocally.

Where it occurs in the wild it is found infrequently in deciduous forests and valleys on acidic loess (i.e. fine, silty soil) with good drainage. The soil it inhabits is typically in the pH range of 5 to 5.5.[12]

In many areas of China it has been long cultivated and it is common in the southern third of the country.[12] It has also been commonly cultivated in North America for over 200 years, but during that time it has never become significantly naturalised.[13]

Taxonomy and naming

The species was initially described by Linnaeus in 1771, the specific epithet biloba derived from the Latin bis 'two' and loba 'lobed', referring to the shape of the leaves.[14] Two names for the species recognise the botanist Richard Salisbury, a placement by Nelson as Pterophyllus salisburiensis and the earlier Salisburia adiantifolia proposed by James Edward Smith. The epithet of the latter may have been intended to denote a characteristic resembling Adiantum, the genus of maidenhair ferns.[15]

The relationship of Ginkgo to other plant groups remains uncertain. It has been placed loosely in the divisions Spermatophyta and Pinophyta, but no consensus has been reached. Since Ginkgo seeds are not protected by an ovary wall, it can morphologically be considered a gymnosperm. The apricot-like structures produced by female ginkgo trees are technically not fruits, but are seeds that have a shell that consists of a soft and fleshy section (the sarcotesta), and a hard section (the sclerotesta).

The ginkgo is classified in its own division, the Ginkgophyta, comprising the single class Ginkgoopsida, order Ginkgoales, family Ginkgoaceae, genus Ginkgo and is the only extant species within this group. It is one of the best-known examples of a living fossil, because Ginkgoales other than G. biloba are not known from the fossil record after the Pliocene.[16][17]
Etymology

The older Chinese name for this plant is 銀果 ('silver fruit'), nowadays pronounced as yínguǒ in Mandarin. The most usual names today are 白果 bái guǒ ('white fruit') and 銀杏 yínxìng ('silver apricot'). The former name was borrowed directly in Vietnamese (as bạch quả). The latter name was borrowed in Japanese (as ぎんなん "ginnan") and Korean (as 은행 "eunhaeng"), when the tree itself was introduced from China.

The scientific name Ginkgo appears to be due to a process akin to folk etymology. Chinese characters typically have multiple pronunciations in Japanese, and the characters 銀杏 used for ginnan can also be pronounced ginkyō. Engelbert Kaempfer, the first Westerner to see the species in 1690, wrote down this pronunciation in his Amoenitates Exoticae (1712) with the "awkward" spelling "Ginkgo". This appears to be a simple error of Kaempfer, taking his spelling of other Japanese words into account, a more precise romanization would have been "Ginkio" or "Ginkjo".[18]

The Ginkgo is a living fossil, with fossils recognisably related to modern Ginkgo from the Permian, dating back 270 million years. The most plausible ancestral group for the order Ginkgoales is the Pteridospermatophyta, also known as the "seed ferns," specifically the order Peltaspermales. The closest living relatives of the clade are the cycads,[19] which share with the extant G. biloba the characteristic of motile sperm. Fossils attributable to the genus Ginkgo first appeared in the Early Jurassic, and the genus diversified and spread throughout Laurasia during the middle Jurassic and Early Cretaceous. It declined in diversity as the Cretaceous progressed, and by the Paleocene, Ginkgo adiantoides was the only Ginkgo species left in the Northern Hemisphere while a markedly different (and poorly documented) form persisted in the Southern Hemisphere. At the end of the Pliocene, Ginkgo fossils disappeared from the fossil record everywhere except in a small area of central China where the modern species survived. It is doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day G. biloba (including G. adiantoides) and G. gardneri from the Paleocene of Scotland.[20]

At least morphologically, G. gardneri and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been ecotypes or subspecies. The implications would be that G. biloba had occurred over an extremely wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the Ginkgo's life-history parameters fit. These are: extreme longevity; slow reproduction rate; (in Cenozoic and later times) a wide, apparently contiguous, but steadily contracting distribution coupled with, as far as can be demonstrated from the fossil record, extreme ecological conservatism (restriction to disturbed streamside environments).[21]

Modern-day G. biloba grows best in environments that are well-watered and drained,[22] and the extremely similar fossil Ginkgo favored similar environments: the sediment record at the majority of fossil Ginkgo localities indicates it grew primarily in disturbed environments along streams and levees.[21] Ginkgo therefore presents an "ecological paradox" because while it possesses some favorable traits for living in disturbed environments (clonal reproduction) many of its other life-history traits (slow growth, large seed size, late reproductive maturity) are the opposite of those exhibited by modern plants that thrive in disturbed settings.[23]

Given the slow rate of evolution of the genus, it is possible that Ginkgo represents a pre-angiosperm strategy for survival in disturbed streamside environments. Ginkgo evolved in an era before flowering plants, when ferns, cycads, and cycadeoids dominated disturbed streamside environments, forming a low, open, shrubby canopy. Ginkgo's large seeds and habit of "bolting" - growing to a height of 10 m before elongating its side branches - may be adaptions to such an environment. The fact that diversity in the genus Ginkgo drops through the Cretaceous (along with that of ferns, cycads, and cycadeoids) at the same time that flowering plants were on the rise, supports the notion that flowering plants with better adaptations to disturbance displaced Ginkgo and its associates over time.[24]

Ginkgo has been used for classifying plants with leaves that have more than four veins per segment, while Baiera for those with fewer than four veins per segment. Sphenobaiera has been used to classify plants with a broadly wedge-shaped leaf that lacks a distinct leaf stem. Trichopitys is distinguished by having multiple-forked leaves with cylindrical (not flattened) thread-like ultimate divisions; it is one of the earliest fossils ascribed to the Ginkgophyta.
Cultivation and uses

Ginkgo has long been cultivated in China; some planted trees at temples are believed to be over 1,500 years old. The first record of Europeans encountering it is in 1690 in Japanese temple gardens, where the tree was seen by the German botanist Engelbert Kaempfer. Because of its status in Buddhism and Confucianism, the Ginkgo is also widely planted in Korea and parts of Japan; in both areas, some naturalization has occurred, with Ginkgos seeding into natural forests.

In some areas, most intentionally planted Ginkgos are male cultivars grafted onto plants propagated from seed, because the male trees will not produce the malodorous seeds. The popular cultivar 'Autumn Gold' is a clone of a male plant.

Ginkgos adapt well to the urban environment, tolerating pollution and confined soil spaces.[25] They rarely suffer disease problems, even in urban conditions, and are attacked by few insects.[26][27] For this reason, and for their general beauty, ginkgos are excellent urban and shade trees, and are widely planted along many streets.

Ginkgos are also popular subjects for growing as penjing and bonsai; they can be kept artificially small and tended over centuries. Furthermore, the trees are easy to propagate from seed.

Extreme examples of the Ginkgo's tenacity may be seen in Hiroshima, Japan, where six trees growing between 1–2 km from the 1945 atom bomb explosion were among the few living things in the area to survive the blast (photos and details). While almost all other plants (and animals) in the area were destroyed, the ginkgos, though charred, survived and were soon healthy again. The trees are alive to this day.

The ginkgo leaf is the symbol of the Urasenke school of Japanese tea ceremony. The tree is the national tree of China.
Culinary use
Ginkgo seeds with sarcotesta removed

The nut-like gametophytes inside the seeds are particularly esteemed in Asia, and are a traditional Chinese food. Ginkgo nuts are used in congee, and are often served at special occasions such as weddings and the Chinese New Year (as part of the vegetarian dish called Buddha's delight). In Chinese culture, they are believed to have health benefits; some also consider them to have aphrodisiac qualities. Japanese cooks add Ginkgo seeds (called ginnan) to dishes such as chawanmushi, and cooked seeds are often eaten along with other dishes.

When eaten in large quantities or over a long period, especially by children[28] the gametophyte (meat) of the seed can cause poisoning by MPN (4'-O-methylpyridoxine). MPN is heat stable and not destroyed by cooking.[28] Studies have demonstrated that convulsions caused by MPN can be prevented or terminated with pyridoxine.

Some people are sensitive to the chemicals in the sarcotesta, the outer fleshy coating. These people should handle the seeds with care when preparing the seeds for consumption, wearing disposable gloves. The symptoms are allergic contact dermatitis[29][30] or blisters similar to that caused by contact with poison ivy. However, seeds with the fleshy coating removed are mostly[clarification needed][quantify] safe to handle.
Ginkgo biloba in Tournai, Belgium
Medicinal uses

Extracts of Ginkgo leaves contain flavonoid glycosides and terpenoids (ginkgolides, bilobalides) and have been used pharmaceutically. Ginkgo supplements are usually taken in the range of 40–200 mg per day. Recently, careful clinical trials have shown Ginkgo to be effective in treating dementia[31] but not preventing the onset of Alzheimer's Disease in normal people.[32][33]
In memory enhancement

Ginkgo is believed to have nootropic properties, and is mainly used as memory[34] and concentration enhancer, and anti-vertigo agent. However, studies differ about its efficacy. The largest and longest independent clinical trial to assess Ginkgo biloba’s ability to prevent memory loss has found that the supplement does not prevent or delay dementia or Alzheimer’s disease.[35] Some controversy has arisen over the conclusions drawn by some studies that were funded by a firm which marketed Ginkgo.[36]

In 2002 a long-anticipated paper appeared in JAMA titled "Ginkgo for memory enhancement: a randomized controlled trial." This Williams College study, sponsored by the National Institute on Aging rather than Schwabe, examined the effects of ginkgo consumption on healthy volunteers older than 60. The conclusion, now cited in the National Institutes of Health's ginkgo fact sheet, said: "When taken following the manufacturer's instructions, ginkgo provides no measurable benefit in memory or related cognitive function to adults with healthy cognitive function."[37] ... The impact of this seemingly damning assessment, however, was ameliorated by the almost simultaneous publication of a Schwabe-sponsored study in the less prestigious journal Human Psychopharmacology. This rival study, conducted at Liberty University, was rejected by JAMA, and came to a very different—if not exactly sweeping—conclusion: There was ample evidence to support "the potential efficacy of Ginkgo biloba EGb 761 in enhancing certain neuropsychological/memory processes of cognitively intact older adults, 60 years of age and over."

According to some studies, Ginkgo can significantly improve attention in healthy individuals.[38][39] In one such study, the effect was almost immediate and reaches its peak in 2.5 hours after the intake.[40]

One study suggests that Ginkgo's effect on cognition may be attributable to its inhibitory effect on norepinephrine reuptake.[41]
In dementia

Ginkgo has been proposed as a treatment for Alzheimer's disease on the basis of positive preclinical results in mice[42] and a 2006 study that found 160 mg of ginkgo extract as effective as a daily 5 mg dose of the cholinesterase inhibitor donepezil in human subjects.[43] A 2008 randomized controlled clinical trial published in the International Journal of Geriatric Psychiatry found gingko ineffective at treating dementia in humans at a daily dose of 120 mg.[44][45] A similar trial published in the same journal in 2010, however, found ginkgo effective at treating mild to moderate dementia at the higher dose of 240 mg daily.[46] Another randomized controlled trial, published in JAMA in 2009, found no benefit from ginkgo in preventing cognitive decline or dementia.[33] A recent meta-analysis of nine studies of ginkgo for use in the treatment of dementia concluded that it was more effective than placebo although, like other dementia drugs, the clinical significance of these moderate effects was difficult to quantify.[31]
In other symptoms

Out of the many conflicting research results, Ginkgo extract may have three effects on the human body: improvement in blood flow (including microcirculation in small capillaries) to most tissues and organs; protection against oxidative cell damage from free radicals; and blockage of many of the effects of platelet-activating factor (platelet aggregation, blood clotting)[47] that have been related to the development of a number of cardiovascular, renal, respiratory and central nervous system disorders. Ginkgolides, especially ginkgolide B, are potent antagonists against platelet-activating factor; and thus may be useful in protection and prevention of thrombus, endotoxic shock, and from myocardial ischeamia.[48]Ginkgo can be used for intermittent claudication.

Ginkgo has been studied as a potential treatment for sexual dysfunction related to SSRI use, but failed to show any effectiveness in placebo-controlled trials.[49][50]

The World Health Organization[51] reports that the medicinal uses of Ginkgo biloba that are supported by clinical data include treatment of the effects mild to moderate cerebrovascular insufficiency [52] as well as the effects of peripheral arterial occlusive diseases.[53] Cerebrovascular insufficiency, i.e., insufficient blood flow to the brain, may manifest itself as such memory deficit, disturbed concentration or headaches. Peripheral arterial occlusive diseases are those in which the blood flow to the smaller arteries are restricted and may include claudication, i.e., painful walking, and Raynaud's disease, a condition in which the extremities such as fingers, toes, nose or ears, feel numb and cold.

Preliminary studies suggested that Ginkgo might be of benefit in multiple sclerosis, but clinical trials failed to show any effect on cognitive function in MS patients.[54]

A study conducted in 2003 by the Department of Dermatology, Postgraduate Institute of Medical Education and Research in Chandigarh, India concluded that Ginkgo is an effective treatment for arresting the development of vitiligo. The study reported that 40 mg of Ginkgo biloba three times per day for 6 months arrested the spread of vitiligo in 20 out of 25 participants in the active group, and induced marked (75% or greater) repigmentation in 10 of these participants.[55]

A later study conducted by University of Toronto researchers and published in 2011 discussed the results of an open label pilot clinical trial of twelve patients taking 60 mg of Ginkgo biloba twice a day for 12 weeks. The study found the progression of vitiligo stopped in all participants; the total VASI (Vitiligo Area Scoring Index) indicated an average repigmentation of vitiligo lesions of 15%. The authors conclude "Larger, randomized double-blind clinical studies are warranted and appear feasible." [56] The Toronto researchers were not sure why the Indian study had better results but they speculated on a number of things. The Indian study was longer; they did not use exactly the same Ginkgo biloba extract; the dosage was different and delivered in an alternate method. They said the genetic variations in the population might have made a difference, as well as diet and social differences. In the Toronto study, some of the participants did not complete the study until November, thus experiencing substantially less sun exposure in the later months of their treatment. Because sun exposure can stimulate the proliferation of melanocytes and vitamin D, this factor could also explain the deviation in results.
Side effects

Ginkgo may have undesirable effects, especially for individuals with blood circulation disorders and those taking anticoagulants such as ibuprofen, aspirin, or warfarin, although recent studies have found that ginkgo has little or no effect on the anticoagulant properties or pharmacodynamics of warfarin in healthy subjects.[57] Ginkgo inhibits monoamine oxidase,[58] and therefore should not be used by people who are taking certain types of antidepressants (monoamine oxidase inhibitors and selective serotonin reuptake inhibitors[59][60]) or by pregnant women, without first consulting a doctor.

Ginkgo side effects and cautions include: possible increased risk of bleeding, gastrointestinal discomfort, nausea, vomiting, diarrhea, headaches, dizziness, heart palpitations, and restlessness.[60][61] If any side effects are experienced, consumption should be stopped immediately.
Allergic precautions and contraindications to use

People taking pharmaceutical blood thinners such as warfarin or coumadin should consult with their doctor before taking Gingko biloba extracts, as it acts as an anti-coagulant.

The presence of amentoflavone in Gingko biloba leaves would indicate a potential for interactions with many medications through the strong inhibition of CYP3A4 and CYP2C9; however, there is a lack of any empirical evidence supporting this. Further, at recommended doses, studies have shown "Multiple-dose administration of Ginkgo biloba did not affect cytochrome P-450 2D6 or 3A4 activity in normal volunteers."[62] It is possible that the concentration of amentoflavone found even in commercial Gingko biloba extracts is too low to be pharmacologically active.
Ginkgo biloba in Morlanwelz-Mariemont Park, Belgium

Ginkgo biloba leaves also contain long-chain alkylphenols together with the extremely potent allergens, the urushiols (similar to poison ivy).[30] Individuals with a history of strong allergic reactions to poison ivy, mangoes, and other urushiol-producing plants are more likely to experience an adverse reaction when consuming Ginkgo-containing pills, combinations, or extracts.
[edit] See also